Electrical stringed instruments require that the string's vibrations be accurately detected and electrically transformed to ultimately produce audible sounds. Presently, the most common form of string transducer on electric guitars and basses is the magnetic "pickup."
Magnetic pickups on guitars date back to 1931. Although they are the dominant form of string vibration transducers on electric guitars today, magnetic pickups are not without disadvantages. Furthermore, because of string-bridge radius and mechanical mounting difficulties, magnetic pickups are limited to certain styles of electric guitars and are not practical for instruments such as violins, harps, and upright bases.
A magnetic pickup operates by generating a magnetic field which is interrupted by vibrations of a steel string. The disadvantages of a magnetic pickup include the problem that non-magnetic strings such as nylon strings cannot be used with a magnetic pickup.
The natural shape of a string's vibration on guitars, basses and other stringed instruments is an ellipse or oval pattern. The magnetic field of the pickup changes the sound and decreases the sustain of a string's vibration by pulling the vibration into a more vertical pattern. Accordingly, another disadvantage of magnetic pickups is that they affect the vibration mode of the string, and therefore the string's natural sound.
Magnetic pickups are primarily sensitive to only the vertical vibrations of a string, and not vibrations in the horizontal axis. This coupled with the effect that magnets have on a string's vibration, causes the string's natural sound to be undesirably "colored."
In cases where the magnet is too powerful or too close to the string, the magnet can produce unwanted harmonic overtones and may actually cause the string to buzz on the instrument's higher frets. Also, the magnetic pickup produces a nonlinear signal which adds further complications to those discussed above.
Most magnetic pickups use pole pieces to conduct magnetic flux up towards the string. Because there is a higher output signal when the string is close to the metal pole, the problems described above are even more pronounced. Furthermore, individual string vibration information cannot be channeled to discreet channels for low-noise conditioning circuitry.
Another type of pickup, the piezoelectric pickup, operates by inducing the dynamic mechanical stress of string vibration on a piezo-crystal. A potential difference appears on opposite ends of the crystal, generating a signal.
Its applications include sensors, microphones and pickups for stringed instruments, such as acoustic style guitars, violins and upright basses. The pickups may be conveniently retrofitted in the bridge of the instrument. Problems arise because the piezo-crystal output impedance is quite high, typically in the mega-ohms. Furthermore, piezoelectric pickups operate on mechanical losses which occur in string and bridge interfaces, therefore, their sound quality is limited.
Piezoelectric pickups have further disadvantages. For example, the response and performance are reduced due to the effect of the mechanical and electrical load on the output voltage. Moreover, the output voltage, which is also frequency dependent, increases with frequency causing the response to rise at higher frequencies. The sound quality is therefore only sufficient at high frequencies.
Optical transducers have been disclosed by Ferber, U.S. Pat. No. 3,733,954, issued May, 1973; by Siebeneiker et al., U.S. Pat. No. 4,563,391, issued January, 1986. The configurations disclosed therein have not been acceptable because of the great difficulty in alignment of the string and sensor during actual use.
Modern guitarists often "bend" strings, moving the string laterally across the fret board. Optical transducers of the prior art do not detect vibration of the strings in both vertical and horizontal axes. Additionally, prior optical transducers have been noisy due to ambient lights and thermal noise, among other reasons. Furthermore, prior optical transducers have not been easily retrofittable due to emitter, lens and sensor alignment problems.